1. Field of the Invention
This invention relates generally to an imprint template with optically detectable alignment marks, such as a template for imprinting patterned-media magnetic recording disks and semiconductor devices, and to a method for making it using directed self-assembly of block copolymers.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric generally circular data tracks. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of spaces between the islands must be destroyed or substantially reduced to render these spaces essentially nonmagnetic. In one type of patterned media, the data islands are elevated regions or pillars that extend above “trenches” and magnetic material covers both the pillars and the trenches, with the magnetic material in the trenches being rendered nonmagnetic, typically by “poisoning” with a material like silicon (Si). In another type of patterned media, the magnetic material is deposited first on a flat disk substrate. The magnetic data islands are then formed by milling, etching or ion-bombarding of the area surrounding the data islands. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer.
One proposed method for fabricating patterned-media disks is by imprinting with a template, sometimes also called a “stamper”, that has a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the template. The patterns on the template may be protrusions (pillars), or recesses (holes), and the type of the pattern is referred to as the polarity of the template. The polymer film receives the reverse image of the template pattern and then, depending on the polarity of the template, either becomes a mask for subsequent etching of the disk substrate if the template is hole-type, or becomes a sacrificial layer for a tone reversal process such as liftoff followed by etching of the disk substrate, if the template is pillar type, to form pillars on the disk in both cases. In one type of patterned media, the magnetic layer and other layers needed for the magnetic recording disk are then deposited onto the etched disk substrate and the tops of the pillars to form the patterned-media disk. In another type of patterned media, the magnetic layers and other layers needed for the magnetic recording disk are first deposited on the flat disk substrate. The polymer film used with imprinting is then pressed on top of these layers. The polymer film receives the reverse image of the template pattern and then becomes a mask, or sacrificial layer for tone reversal, for subsequent milling, etching or ion-bombarding the underlying layers.
The template may be a master template for directly imprinting the disks. However, the more likely approach is to fabricate a master template with a pattern of pillars corresponding to the pattern of pillars desired for the disks and to use this master template to fabricate replica templates using imprint lithography. The replica templates may have the opposite or same polarity of the master template. The replica templates are then used to directly imprint the disks.
The making of a master imprint template is a difficult and challenging process. The use of electron beam (e-beam) lithography using a Gaussian beam rotary-stage e-beam writer is viewed as a possible method to make a master template capable of imprinting patterned-media disks with a track pitch (island-to-island spacing in the radial or cross-track direction) of about 35 nm, and an island pitch (island-to-island spacing in the circumferential or along-the-track direction) of about 35 nm. These dimensions generally limit the areal bit density of patterned-media disks to about 500 Gbit/in2. To achieve patterned-media disks with an ultra-high areal bit density greater than 1 Terabits/in2, a track pitch and an island pitch of about 20 nm will be required. However, a template capable of imprinting patterned-media disks with these small dimensions over an area equal to the active area of a disk may not be practical with the resolution of e-beam lithography.
Directed self-assembly of block copolymers (BCPs) has been proposed for making the template and is believed capable of achieving areal bit densities of greater than 1 Terabit/in2. U.S. Pat. No. 7,976,715 B2, assigned to the same assignee as this application, describes the use of directed or guided self-assembly of block copolymers to form a pattern of generally radial lines on a template substrate, followed by conventional lithography to form a pattern of concentric generally circular rings over the radial lines. After etching of the substrate and removal of resist, the substrate has a pattern of protrusions of the other block copolymer component, which are then used as an etch mask to etch the substrate into a pattern for imprinting disks with discrete data islands arranged in concentric generally circular data tracks.
Another method to generate a master template containing a pattern of pillars corresponding to the pattern of data islands is to combine the patterns from two submaster templates using separate imprint steps. Each imprint submaster template is created using directed self-assembly of BCPs, and may also require line-doubling techniques for even smaller pattern size and higher density. Pending application Ser. No. 13/627,492, filed Sep. 26, 2012 and assigned to the same assignee as this application, describes the use of two such imprint submaster templates, one with a pattern of generally radial spokes or lines, and the other with generally concentric rings, to make the master template by two separate imprinting steps with the two submaster templates.
Proper alignment during an imprint step between the imprint template and substrate is often necessary. In the case of imprinting from a template to a disk, it is necessary to ensure that the imprinted pattern from the imprint template is concentric with the disk substrate because the eccentricity between the data pattern and the disk substrate on a finished media increases the repeatable runout (RRO), a factor which can cause the head to go off-track during the operation of the disk drive. In the case of using two submaster templates to create one master template, it is necessary for each submaster template to be aligned with the substrate of the master template during the two separate imprint steps to ensure the complete overlap of the patterns of rings and radial spokes, and formation of islands and servo patterns at the proper locations in all tracks.
Imprint templates have also been proposed for use in semiconductor manufacturing. For example, imprint templates can be used to pattern parallel generally straight lines in MPU, DRAM and NAND flash devices. Imprinting has been proposed and placed on the roadmap for making these devices below the node of 16 nm feature size. The semiconductor devices are typically silicon wafer based, and are manufactured in a sequence of steps, each stage placing a pattern of material on the wafer. In this way transistors, contacts, etc., all made of different materials, are laid down. In order for the final devices to function correctly, these separate patterns must be aligned correctly. For example, contacts, lines and transistors must all line up. Misalignment of any kind can cause short circuits and connection failures, which in turn impact fabrication yield and profit margins. The control of such pattern-to-pattern alignment is also referred to as “overlay control”.
For these reasons, imprint templates are often required to have optically-detectable alignment marks located in the non-active areas outside the active area of the template, i.e., the area of the template containing the protrusions or recesses that corresponds to the area on an imprinted substrate containing structures serving functional purposes, such as data islands or servo features on a disk. The alignment marks typically serve no functional purposes on finished devices. When alignment is necessary, the substrate also needs to have corresponding alignment marks or a specific geometry (such as the center hole in a disk). The alignment marks on a template or substrate may be optically detectable. Optical detection is a known method, and can be done using scatterometry (such as sensing the diffracted or reflected light signal from grating-based alignment marks or certain geometry), or through image recognition (such as directly reading caliper or protractor shaped alignment marks, or detecting Morié patterns resulting from two overlapped grids with different pitch or orientation). In one approach to align the template and substrate during imprinting, the imprint system optically detects the alignment marks on the template, and alignment marks or certain geometry on the substrate to determine their coordinates (locations), applies the imprint resist to the substrate, then brings the template and substrate to a common coordinate for contact and presses them together to complete the imprinting. In a more sophisticated approach, the imprint system first applies the resist to the substrate, brings the template and substrate almost in contact, then determines the misalignment by analyzing optical signals from alignment marks on both the template and substrate and actively adjusts their relative positions to reduce the misalignment within a set specification, before finally pressing the template and substrate together to complete the imprinting.
On an imprint template fabricated using directed self-assembly of BCPs, the dimensions and pitches of the template features are determined by the natural pitch (L0) of the BCPs, which are typically in the range of 8-60 nm. However, due to the optical diffraction limit, optically detectable alignment marks need to have dimensions and pitches in the order of the wavelength of the detection light source, or larger. To avoid premature exposure of the photosensitive imprint resist to a curing light, the light source typically has a wavelength between 500 nm to a few microns. Typical alignment marks have dimensions of at least a micron. Thus directed self-assembly of BCPs used to form high density patterns with pitches less than 60 nm in the active area is not capable of simultaneously forming alignment marks with dimensions and pitches in the micron range. If a separate step is used to form the alignment marks in addition to the directed self-assembly of BCPs, the alignment marks will lose their intrinsic self-alignment with the BCP patterns in the active area, resulting in a build-in misalignment at the template level which propagates to all imprinted substrates.
What is needed is a imprint template and a method for making it using self-assembly of BCPs that can result in the simultaneous formation of the desired high density patterns in the active area and the required alignment marks in the non-active area.